There are primarily two types of pre-distortion techniques used to linearize power amplifiers: digital and analog. Digital techniques typically use envelope detectors, analog-to-digital converters (ADCs), look-up-tables, digital-to-analog converters (DACs), and vector modulators to create the pre-distorted waveforms to be applied to the power amplifier. Within reason, a multitude of pre-distorted waveforms can be created with this type of technique. However, such digital techniques include an inherent throughput delay associated with the ADC, digital signal processing, and DAC circuits. Most of this delay is related to the clock speed of operation, since each of these circuits is typically associated with many clock cycles of delay. To fundamentally reduce the delay, the clock speed must be increased. Additionally, digital techniques usually require expensive ADC and DAC components in the signal processing chain.
Existing analog predistortion circuits generally have fundamental limitations, since precise amplitude/phase relationships are not easily derived. Most analog circuits used are diode-or transistor-based circuits that do not have independent gain and phase relationships or controls. This means that a particular non-linear element is used for pre-distortion and is merely an approximation to the ultimate desired transfer curve. For example, a particular non-linear element, e.g., a transistor, is used as the pre-distortion device. The device may be driven hard enough to create the desired amplitude compression that matches the power amplifier's compression curve. However, as the pre-distorter is driven hard, the phase also compresses, but to a degree that may or may not match the power amplifier's phase-compression curve, resulting in a mismatch between the pre-distorter and the power amplifier, reducing the effectiveness of the pre-distorter. If the phase does match, it is only under very unique conditions that might not apply to another power amplifier's transfer curve, producing repeatability problems. As a result, most analog pre-distorters produce inadequate approximations to the amplifier's compressive characteristics.